Recent remarkable progress of sugar-chain science has clarified some of its physiological role, which makes it possible to develop pharmaceuticals and functional materials based on oligosaccharides possessing physiological activities. However, only limited types of oligosaccharides are currently available on the market, and in addition, they are extremely expensive. Moreover, those oligosaccharides can be produced only on a reagent level, and a mass-production method for them has not yet been fully established.
Conventionally, oligosaccharides have been produced by way of extraction from natural substances, chemical synthesis, enzymatic synthesis, or a combination of these. Among these processes, enzymatic synthesis has been considered the best suited for the mass-production for the following reasons: (1) enzymatic synthesis does not require intricate procedures such as protection and removal of protection, which are required for chemical synthesis, (2) substrate specificities of enzymes enable the synthesis of oligosaccharides having highly structural specificities. In addition, recent progress in recombinant DNA technology have made it possible to mass-produce various types of enzymes economically and in large quantities, which also contributes to establishing the superiority of enzymatic synthesis over other processes.
Two processes for the synthesis of oligosaccharides through use of enzymatic synthesis are available: a process that makes use of the reverse reaction of a hydrolase, and a process that makes use of a glycosyltransferase. The former has an advantage that it can employ inexpensive monosaccharides as the substrate, but as it employs the reverse reaction to the decomposition reaction, it is not necessarily the best process for the synthesis of oligosaccharides in terms of the yield and application to oligosaccharides possessing a complicated structure.
On the other hand, the latter makes use of a glycosyltransferase and has an advantage over the former in terms of the yield and application to the synthesis of oligosaccharides possessing a complicated structure. Moreover, the mass-production of various types of glycosyltransferase enabled by recent progress in recombinant DNA technology also contributes to realization of this process.
However, sugar nucleotides, which are sugar donors used in a synthesis that makes use of a glycosyltransferase, are with few exceptions still expensive, and are provided only in small amounts on reagent levels. There has been reported a process for preparing uridine diphosphate-N-acetylglucosamine (UDPAG), which is a donor of N-acetyl glucosamine, by use of an osmolarity-resistant yeast (Japanese Patent Application Laid-Open (kokai) No. 8-23993), but problems still remain to be solved before its industrial production is realized.